<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .slider-bar {
            position: absolute;
            left: 50%;
            top: 350px;
            margin-left: 600px;
            width: 45px;
            height: 130px;
            background-color: pink;
        }
        
        .w {
            width: 1200px;
            margin: 10px auto;
        }
        
        .header {
            height: 150px;
            background-color: purple;
        }
        
        .banner {
            height: 300px;
            background-color: skyblue;
        }
        
        .main {
            height: 1000px;
            background-color: yellowgreen;
        }
        
        span {
            display: none;
            position: absolute;
            bottom: 0;
        }
    </style>
</head>

<body>
    <div class="slider-bar">
        <span class="goBack">返回顶部</span>
    </div>
    <div class="header w">头部区域</div>
    <div class="banner w">banner区域</div>
    <div class="main w">主体部分</div>
    <script>
        var sliderbar = document.querySelector('.slider-bar');
        var banner = document.querySelector('.banner');
        var bannerTop = banner.scrollTop;
        var sliderbarTop = sliderbar.offsetTop - bannerTop;
        var main = document.querySelector('.main');
        var goBack = document.querySelector('.goBack');
        var mainTop = main.offsetTop;
        document.addEventListener('scroll', function() {
            // console.log(pageYOffset);
            if (window.pageYOffset >= bannerTop) {
                sliderbar.style.position = 'fixed';
                sliderbar.style.top = sliderbarTop + 'px';
            } else {
                sliderbar.style.position = 'absolute';
                sliderbar.style.top = '300px';
            }
            if (window.pageYOffset >= mainTop) {
                goBack.style.display = 'block';
            } else {
                goBack.style.display = 'none';
            }
        })
        goBack.addEventListener('click', function() {
            animate(window, 0)
        });

        function animate(obj, target, callback) {
            clearInterval(obj.timer);
            obj.timer = setInterval(function() {
                if (window.pageYOffset == target) {
                    clearInterval(obj.timer);
                    // if (callback) {
                    //     callback();
                    // }
                    callback && callback();
                }
                var step = (target - window.pageYOffset) / 10 > 0 ? Math.ceil((target - window.pageYOffset) / 10) : Math.floor((target - window.pageYOffset) / 10);
                // obj.style.left = window.pageYOffset + step + 'px';
                window.scroll(0, window.pageYOffset + step);
                // obj.style.left = obj.offsetLeft + 1 + 'px';
            }, 15);
        }
    </script>
</body>

</html>